The separation of liquid or gaseous mixtures of substances with the aid of solid materials has been practised commercially and industrially for decades for a plurality of applications. Zeolites are being used more and more here due to their outstanding selectivities.
The zeolites are generally applied to carriers. The use of granules, moulded bodies or membranes is prior art. Inert solids or polymers, which together with the zeolites produce sorptively acting composite materials, are suitable as binding agents.
An example of this type of composite material is described in EP 0 773 829. Here a hydrophobic molecular sieve with a pore diameter of 5.5-6.2 angstrom is embedded into fibrillated polytetrafluoroethylene (PTFE) or blown microfibres (polyamide, polyester, polyurethane, polyolefin etc.) in a ratio of 40:1 to 1:40 as a selective sorption medium. The processing takes place with the aid of a liquid lubricant. The doughy mass is calendered biaxially, by means of which a porous film is finally produced after drying, the porosity of which can be derived from the amount of lubricant. Similar composite materials are described in the patents U.S. Pat. No. 4,153,661, U.S. Pat. No. 4,460,642 and U.S. Pat. No. 5,071,610 which likewise describe porous fibrous membranes based on PTFE for the addition of sorbents or catalytically effective particulate substances.
PTFE is particularly suitable as a matrix polymer because it can be fibrillated, is thermally stable, chemically inert and hydrophobic, i.e. it can be processed to form stable and highly flexible fibre fleeces, can be used in the working temperature range of −250 to +260° C., neither absorbs water nor is soluble; in addition, PTFE is largely inert with respect to acids and lyes. PTFE is partially crystalline and can be fibrillated above the phase transition temperature of 19° C., i.e. by applying shearing forces to PTFE power or the PTFE balls contained in the dispersion, the crystallites contained in the material can be uncoiled to form thin filaments (this effect can be observed even better above 30° C.; this is where the second phase transition of PTFE takes place). These filaments, in the best case only a few molecule layers thick, are capable, using an appropriate processing technique, of extending around, embedding and holding large quantities of filler, by means of which high-grade cross-linked, highly filled PTFE filler composites are obtained. Moreover, the polymer fibres hook and loop onto one another during the shearing, and this gives the composite material a certain degree of mechanical stability. The ability to be processed into films and moulded bodies can, however, be greatly hindered by the strong deformation forces required depending on the filler, and this is why it has proved best to use, wherever possible, lubricants (water, alcohols, crude oil distillates, hydrocarbons and other solvents) which facilitate the processing process, support the fibrillation and prevent premature destruction/tearing of the fibres due to excessive shearing. After the shaping, the solvent that has been added is generally eliminated by heating, by means of which an additional defined degree of porosity remains. By means of an optional sintering process of the PTFE material at temperatures of around 330° C., but below 360° C. (start of decomposition) the composite material obtains its final stability and shape.
Examples of the area of application for these composite materials are the membrane methods pervaporation or vapour permeation with so-called Mixed Matrix Membranes (MMMs), but also various adsorption methods, such as so-called Solid Phase Extraction (SPE) or drying methods.
Solid phase extraction is to be understood as meaning the physical separating process between a fluid and a solid phase, the component to be isolated and analysed being dissolved in a liquid or gaseous solvent. SPE has an application, for example, in analytical or preparative chromatography (e.g. High Performance Liquid Chromatography, HPLC, or Gas Chromatography, GC).
For drying methods, bulk composed of adsorption materials which adsorb water as the fluid flows through the bulk, is generally used. The best known example is the dehydration of ethanol with the aid of the hydrophilic zeolites 3A, 4A, 5A or 13X.
It is known from EP 0 773 829 that organic components from a fluid, i.e. from a liquid or a gas, can also be adsorbed.
So as to subsequently extract the components in enriched form, i.e. more highly concentrated or pure form, the organic components must be desorbed. For desorption there are the following possibilities:
First of all, the adsorbed components can be expelled by other components. However, it is a disadvantage of this method that after desorption the adsorption means is charged with the components used for the expulsion, and so further steps are required in order to remove the latter.
Secondly, the temperature of the adsorption means can be increased until the adsorbed components desorb thermally. This possibility is mentioned in EP 0 773 829. It is a disadvantage of this method, however, that as the size of the adsorber columns increases, it becomes harder and harder to introduce the heat via the adsorber walls because the wall surface to volume ratio becomes lower and lower and so less favourable. The heating of the adsorption means by means, for example, of hot flushing gas, is also associated with very large volumetric flows of the flushing gas due to the low heat capacity of gases. In addition, by means of this so-called Temperature Swing Adsorption method (TSA) short cycle times cannot generally be achieved because heating and cooling can last a very long time.
The third possibility is the so-called Pressure Swing Adsorption method (PSA) wherein the adsorbed components are desorbed by reducing the pressure. The advantage of this method is that the pressure drops very quickly and evenly in the whole adsorption column and can then be raised again. This makes very short cycle times possible. Short cycle times make it possible to reduce the required amount of adsorption means so that the adsorber columns can have clearly smaller dimensions. This reduces not only the costs for the adsorption means and the investment costs for the columns, but also reduces the operating costs of the adsorption units because shorter adsorber beds also give rise to smaller pressure losses as the gaseous mixture of substances flows through the adsorber bulk, and this must be overcome by pumps or compactors.
The disadvantage of the PSA methods, however, is that the adsorption means is subjected to frequent pressure swings, and this leads to mechanical loading of the adsorption means. The adsorption means is typically bulk composed of solid particles which rub against one another with each pressure swing so that friction occurs. Cavities formed upon granulation or deformation of the adsorption means can lead to the granules or moulded bodies breaking when there are pressure fluctuations. Furthermore, the particles may burst or break up. This can reduce the life of the adsorption means and further increase the pressure loss due to the decreasing average particle size. In order to reduce these undesired effects as far as possible, two method versions are essentially used:
(a) The pressure swings are implemented as slowly as possible, i.e. the vacuum pump is only started up slowly when tension is released, and when the pressure is increased the gaseous mixture of substances only flows into the adsorber column slowly. Accordingly, these times for the pressure swings are not available at the target pressure for adsorption or desorption. The fact that adsorption below the target pressure and desorption above the target pressure proceed less efficiently, i.e. less substance is adsorbed or desorbed per unit of time, must be compensated for by extending the cycle time. The time efficiency of the whole method is therefore disadvantageously reduced.
(b) The cycle time is extended by extending the adsorption phase and/or the desorption phase in order to reduce the number of pressure swings and so to reduce the number of procedural steps associated with strong mechanical loading of the adsorber material. However, each extension of the adsorption phase and/or of the desorption phase requires greater amounts of adsorption means, and so this has a negative impact on the cost efficiency of the method.
Therefore, the mechanical loading of the adsorption material during the pressure swing leads to a reduction of the efficiency of the method.
In summary it can be established that an optimal PSA method should use an adsorption means which has low pressure losses and makes short cycle times possible without this having any negative impact on the life of the adsorption means.